1. Field of the Invention
The present invention relates to a radio-card communications system which comprises a radio card for transmitting data stored in it, in response to a question signal, and a card reader for generating and sending a question signal to the radio card.
2. Description of the Related Art
A radio-card communications system is known in which signals are exchanged between a card reader and a radio card. This system is schematically illustrated in FIG. 1. As FIG. 1 shows, the system comprises a card reader 1A and a radio card 1B.
The card reader 1A comprises a modulator 11, a transmitter 12, a signal-extracting device 13, an antenna 14, a receiver 15 and a demodulator 16. The modulator 11 modulates a carrier with a question signal (data). The transmitter 12 power-amplifies the modulated carrier output from the modulator 11. The power-amplified carrier is supplied via the signal-extracting device 13 to the antenna 14. The antenna 14 sends the carrier toward the radio card 1B. The antenna catches a modulated response signal (data) transmitted from the radio card 1B. The demodulated response signal is supplied via the signal-extracting device 13 to the receiver 15. The receiver 15 detects and supplies the response signal to the demodulator 16. The demodulator 16 demodulates the response signal.
The radio card 1B comprises an antenna 17, a signal-extracting device 18, a receiver 19, a demodulator 20, a modulator 21 and a transmitter 22. The antenna 17 receives a carrier signal modulated with a question signal that is transmitted from the card reader 1A. The modulated carrier is supplied via the signal-extracting device 18 to the receiver 19. The receiver 19 detects the carrier. The demodulator 20 demodulates the carrier, thus obtaining the question signal. The modulator 21 modulates a carrier with a response signal read from a memory (not shown). The transmitter 22 power-amplifies the modulated carrier. This carrier is supplied via the signal-extracting device 18 to the antenna 17, which sends the carrier modulated with the response signal, toward the card reader 1A.
The radio-card communications system of FIG. 1 uses a carrier having a frequency f1 to send data from the card reader 1A to the radio card 1B, and a carrier of a frequency of f2 to send data from the radio card 1B to the card reader 1A. That is, the system is a duplex system wherein two carriers of different frequencies are utilized--one in transmitting signals, and the other in receiving signals. Thanks to the difference in frequency between the transmitting carrier and the receiving carrier, the system shown in FIG. 1 is advantageous in some respects. First, it can employ various data transmission schemes applicable to radio-card communication. Second, data items can be transmitted in two directions at the same time.
There is a problem with the radio card 1B, however. The transmitter 22 incorporates a power amplifier. The radio card 1B inevitably consumes much power and needs to be equipped with a large battery (not shown). The larger the battery, the larger and heavier the radio card 1B. This runs counter to the demand that the radio card be as small and light as possible to increase its portability.
To solve this problem, a radio-card communications system has been developed which comprises a radio card having no power amplifiers as illustrated in FIG. 2. The components of this system, which are identical to those shown in FIG. 1, are denoted at the same reference numerals in FIG. 2 and will not be described in detail in the following explanation.
The radio-card communications system of FIG. 2 works exactly in the same way as in the system of FIG. 1 in order to transmit data from the card reader 1A to the radio card 1B. To transmit data the other way around, the card reader 1A sends a carrier having a frequency f1, not modulated, to the radio card 1B. In the radio card 1B, the carrier received by the antenna 17 is supplied via the signal-extracting device 18 to the modulator 21. The modulator 21 modulates the carrier with a response signal. The modulated carrier is supplied, without being power-amplified, through the signal-extracting device 18 to the antenna 17.
Having no power amplifier, the radio card 1B used in the communications system of FIG. 2 consumes only a little power. Hence, the radio card 1B only needs a small battery. A problem arises, however, because carriers of the same frequency f1 are used to transmit signals from the card reader 1A to the radio card 1B and vice versa. In the card reader 1A, part of a signal drifts from the transmitting section into the receiving section, adversely influencing the processing of the signal received.
The signal received by the card reader 1A has the high-frequency spectrum roughly sketched in FIG. 3A. It has the frequency spectrum roughly shown in FIG. 3B after it has been detected. In FIGS. 3A and 3B, frequencies are plotted on the abscissa, and signal magnitudes on the ordinate. As can be seen from FIGS. 3A and 3B, there is a small frequency difference between the carrier C and the modulated component S. It is difficult to separate the carrier C and the component S. If part of a signal drifts from the transmitting section into the receiving section and the received signal is at a low level, the modulated component S of the received signal cannot be detected. It is therefore necessary to set the received signal at a high level. To this end, the card reader 1A and the radio card 1B cannot be located at a long distance.